Glass is widely used as a transparency in a variety of applications due to its superior optical qualities. For example, glass is commonly used as a glazing material or as an architectural material for buildings. Glass is also commonly used as a transparency in vehicular applications. Unfortunately, glass is a relatively dense material and is also relatively brittle such that relatively large thicknesses are required to provide sufficient strength for resisting shattering when the glass is impacted by an object such as a projectile.
In attempts to avoid the weight penalty associated with glass, transparencies may be fabricated from polymeric materials. For example, transparencies may be formed of optically transparent monolithic polymers such as acrylic which is less dense than glass and which possesses suitable optical properties. Unfortunately, acrylic is a relatively low strength material making it generally unsuitable for many applications where high impact resistance is required.
In consideration of the weight penalties associated with glass and the strength limitations of monolithic polymers, manufacturers have also fabricated transparencies from polymeric materials reinforced with glass fibers. The glass fibers may be embedded within an organic and/or polymeric matrix to provide improved strength and impact resistance. Unfortunately, the addition of glass fibers to the polymeric matrix may undesirably affect the optical quality of the transparency. For example, the glass fibers may have a cylindrical configuration causing each glass fiber to act as a small lens. The cumulative effect of the plurality of glass fibers is a scattering of light as the light passes through the transparency such that objects viewed through the transparency may appear blurred.
In attempts to avoid the scattering of light caused by cylindrically-shaped glass fibers, manufacturers may fabricate the fibers in a ribbon shape having an elongated cross-section with generally planar upper and lower surfaces. In a given layer, such fibers are typically spaced apart from one another resulting in some of the incident light passing between the fibers without going through the fibers. When there is a mismatch in the refractive index of the materials, there is a deleterious effect on the optics of the transparency due to the more rapid phase advance of a light wave of the incident light when the wave front passes through the material having a higher refractive index. The consequence of the incident plane wave of light is that the wave front will become distorted and lead to optical scatter and blurring when an image is formed. The cumulative effect in a multi-layer composite panel is that an incident wave front will become progressively more distorted as the wave front passes through an increasing number of layers of the transparency. The greater the quantity of layers in the transparency, the greater the amount of optical distortion in the wave front resulting in greater optical scatter and blurring.
A further drawback associated with flat or ribbon-shaped fibers is that the side surfaces of the fibers may be rounded. Unfortunately, the rounded side surfaces result in unwanted refractive wave steering of the light which causes significant optical distortion when the refractive index of the fiber is different than the refractive index of the matrix. Manufacturers may also fabricate the fibers with squared-off side surfaces oriented generally perpendicular to the planar upper and lower surfaces. Unfortunately, when the side surfaces are viewed off angle, differences in refractive index of the fibers and matrix will result in optical distortion due to refractive and diffractive effects.
Although the fibers and the matrix may be selected to have generally matched refractive indices at a given temperature, changes in temperature of the composite article may result in differences in refractive index if the fibers and matrix have different temperature coefficients of refractive index. Furthermore, the refractive index of the fibers and matrix may differ as a result of residual stresses that may be induced in the fibers or matrix during manufacturing.
As can be seen, there exists a need in the art for a transparent composite article having a fiber configuration that provides improved optical performance over a wide temperature range despite difference in refractive index of the fibers and the matrix.